This invention relates to improvements in fasteners generally and in particular to screws or the like having recesses which are suited to be driven with powdered drivers. Among the primary difficulties which have been presented in connection with the driving of screws has been to control the torque to which such screws are tightened by the driver. The difficulties are amplified when power drivers are used. Prior to the development of power drivers with clutch devices, which may be preset to control the torque applied to the screw, the most widely used technique for controlling the torque was to design the shape of the driver-receptive recess and the driver so that they would disengage when a predetermined torque was reached. For example, U.S. Pat. No. 2,046,837 to Phillips shows such a recess in which the walls of the recess are inclined so as to impart an axial removal force to the driver as the applied torque increases, a characteristic referred to as "cam-out". While this was generally effective, it required some measure of skill by the operator in that in order for the driver to cam-out of the recess at the desired torque, the operator would have to control the amount of axial force of the driver and would also have to maintain the driving tool in substantially axial alignment with the screw. Because of the operator-controlled variables, it was not uncommon for power driven screws to be over-torqued or under-torqued. In addition, disengagement of the driver would cause the driver to slip out of the recess and sometimes mar the surface of the work.
With the development of improved clutches for power drivers to set and control the maximum applied torque, it became desirable to provide improved recesses which could be engaged by the driver more firmly to retard futher the tendency to cam-out. To this end, efforts were made to design the driver-engaged driven surfaces of the recess so that they would be as vertical (parallel to the screw axis) as possible to avoid the camming effect of upwardly and outwardly inclined drive walls. For example, U.S. Pat. Nos. 3,237,506 and Re. 24,878 illustrate recesses having substantially vertical drive walls.
While a recess having vertical drive walls is generally effective to retard cam-out along the axis of the screw when the driver is in axial alignment with the axis of the screw, there still is a tendency for the driver to slip out of engagement from the recess when the driver is out of axial alignment with the screw. This results from the fact that when the driver axis is out of alignment with the screw axis, the sufaces of the driver become inclined with respect to the recess surfaces which, when the screw is driven tends to urge the driver out of the recess. Thus, a certain degree of skill still is required even when using a power driver having a slip clutch in that the driver must be maintained in axial alignment with the screw in order to fully achieve the benefits of the vetical walled recess. However, even skilled operators, by reason of fatigue or for other reasons, sometimes may have difficulty in maintaining the driver and screw in axial alignment. It is among the objects of the invention to provide an improved recess configuration which retards the tendency of the driver to pop-out of or otherwise become disengaged from the screw recess even when there is some misalignment between the driver and the screw.
Also among the prior considerations relating to recessed screws is the characteristic which is commonly referred to as "stick-fit", a condition in which the recess and driver fit together so that the screw will cling to the driver when they are mated. This is desirable in those instances where it is an advantage to the operator to attach the screw to the driver so that he may then position the screw by the driver and then simply drive the screw into the workpiece. Such a stick-fit often is difficult to achieve with consistency because of variations in manufacturing tolerances. Also, drivers and recess-forming punches wear with use which may result in somewhat less than an ideal stick-fit between a particular screw and driver. Moreover, there are instances in which, while a stick-fit is an initially desirable characteristic, it may become undesirable if the fit is so tight that it tends to maintain the stick-fit after the screw has been driven. In this regard, many drivers hold the driver bit in a socket by a magnet which, if the stick fit is retained after the screw has been driven, results in the bit inconveniently remaining stuck to the screw after it has been driven as the magnets employed usually retain the bit only under a light force. It is among the objects of the invention to provide a screw recess having stick-fit characteristics and in which the force employed in driving the screw deforms those regions of the recess which provided the stick-fit, thus destroying the stick-fit and assuring easy removal of the driver bit.